This invention relates to a steam generating system and, more particularly, to a sub-critical or super-critical once-through steam generating system for converting water to steam.
In general, a once-through steam generator operates to circulate a pressurized fluid, usually water, through a steam generating section and a superheating section to convert the water to steam. In these arrangements, the water entering the unit passes once through the circuitry and discharges from the superheating section outlet of the unit as superheated steam for use in driving a turbine, or the like.
These arrangements provide several improvements over conventional drum-type boilers, and although some problems arose in connection with early versions of the once-through generators, such as excessive startup thermal losses, mismatching of steam temperature, the requirement for sophisticated controls and additional valving during startup, these problems have been virtually eliminated in later generating systems.
For example, the system disclosed in U.S. Pat. No. 4,099,384 and assigned to the assignee of the present application, includes a plurality of separators disposed in the main flow line between the steam generating section and the superheating section and adapted to receive fluid flow from the steam generating section during startup and full load operation of the system. This arrangement enables a quick and efficient startup to be achieved with a minimum of control functions, and with minimal need for costly valves. Also, the turbines can be smoothly loaded at optimum pressures and temperatures that can be constantly and gradually increased without the need of boiler division valves or external bypass circuitry for steam dumping. Also, according to this system, operation can be continuous at a very low load with a minimum of heat loss to the condenser.
In the latter arrangement, the walls of the furnace section of the generator are formed by a plurality of vertically extending tubes having fins extending outwardly from diametrically opposed portions thereof, with the fins of adjacent tubes being connected together to form a gas-fight structure. During startup, the furnace operates at constant pressure and super-critical water is passed through the furnace boundary walls in multiple passes to gradually increase its temperature. This requires the use of headers between the multiple passes to mix out heat unbalances caused by portions of the vertically extending tubes being closer to the burners than others or by the tubes receiving uneven absorption because of local slag coverage, burners being out of service, and other causes. The use of these intermediate headers, in addition to being expensive, makes it undesirable to operate the furnace at variable pressure because of probability of separation of the steam and liquid within the header and uneven distribution to the downstream circuit. Therefore, this type of arrangement requires a pressure reducing station interposed between the furnace outlet and the separators to reduce the pressure to predetermined values and, in addition, requires a relatively large number of downcomers to connect the various passes formed by the furnace boundary wall circuitry.
U.S. Pat. No. 4,178,881, also assigned to the present assignee discloses a steam generator which incorporates the features of the system discussed above and yet eliminates the need for intermediate headers, additional downcomers, and a pressure reducing station. Toward this end, the boundary walls of the furnace section of the latter steam generator are formed by a plurality of interconnected tubes, a portion of which extend at an acute angle with respect, to a horizontal plane. In this arrangement, the boundary walls defining the upper and lower portions of the furnace section of the steam generator are formed by vertical tube portions and the intermediate portion of the furnace section are formed by angular tube portions.
One geometric consequence of the angular tube arrangement is that, with reference to a horizontal plane, one angular tube generally occupies the space of two or three vertical tubes (depending on the angle of the angular tubes). Making the transition between the vertical and the angular tubes has typically been addressed using a bifurcated fitting (connecting one angular tube to two or three vertical tubes), an intermediate transition header, or a spiral-wound hopper (in the lower portion of the furnace).
Although these methods for making the transitions between the vertical and angular tube portions are effective, they have disadvantages relating to seals, two-phase flow distribution, thermo-hydraulic sensitivity, structural integrity, and thermal fatigue longevity in cycling service.